Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,338.6	$ 1,200.8
Net Investment Income	100.0	103.1
Change in Value of Alternative Energy Partnership Investments	(16.7)	(15.4)
Other Income (Loss)	2.4	1.5
Total Revenues	1,388.7	1,352.0
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(28.2)	52.2
Net Impairment Losses Recognized in Earnings	(8.9)	(4.0)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,420.4	1,288.4
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,021.6	877.6
Net Investment Income	34.9	35.0
Change in Value of Alternative Energy Partnership Investments	(8.4)	(7.3)
Other Income (Loss)	1.7	0.9
Total Revenues	1,049.8	906.2
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	155.6	162.2
Net Investment Income	12.5	15.9
Change in Value of Alternative Energy Partnership Investments	(3.9)	(4.1)
Total Revenues	164.2	174.0
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	161.4	161.0
Net Investment Income	49.4	51.1
Change in Value of Alternative Energy Partnership Investments	(4.4)	(4.0)
Other Income (Loss)	0.0	0.1
Total Revenues	206.4	208.2
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(28.2)	52.2
Gain (Loss) on Sale of Investments	1.5	13.8
Net Impairment Losses Recognized in Earnings	(8.9)	(4.0)
Other	$ 3.9	$ 1.6